1. Field of the Invention
The present invention relates to a fluxing agent for a metal cast/joint, more to a method for flux joining aluminum components in a mold.
2. Prior Art
The fabrication of large aluminum structures has traditionally been in an assembly process wherein an assortment of parts are joined together by welding, riveting, bolting, adhesive bonding or the like. Each of these processes are labor intensive and are often difficult to accomplish for the geometries of certain components. For example, welding of components to form a large structure is problematic because the components must be made to stringent tolerances to ensure proper mating between the components, machining operations to achieve these tolerances must be carefully controlled to achieve consistent component fit and size of the welds. The assembly fixtures, welding power sources and welding process steps are costly.
One alternative to assembling numerous parts has been casting. Casting process have been developed to reduce costs and improve repeatability as well as consistency of the assemblies. Casting processes typically eliminate the number of parts and reduce the assembly steps of fabricating a large structure.
Casting of molten metal onto an extruded aluminum member is disclosed, for example in U.S. Pat. No. 5,273,099. A flux including potassium and fluorine is applied to the extruded aluminum member. Molten aluminum alloy is poured into a mold containing the flux coated aluminum member. Upon solidification, a joint forms between the cast aluminum and the flux coated aluminum member. While potassium and fluoride base fluxes may be used to cast join aluminum components, the strengths of the bonds between the components have been insufficient.
Accordingly, a need remains for a flux for cast joining aluminum components with a metallurgical bond that has the strength of a brazed or soldered joint.
This need is met by the method of the present invention of joining an aluminum cast member to an aluminum component. The method includes the steps of coating a surface of an aluminum component with flux comprising cesium fluoride, placing the flux coated component in a mold, filling the mold with molten aluminum alloy and allowing the molten aluminum alloy to solidify thereby joining a cast member to the aluminum component. The flux preferably includes aluminum fluoride and alumina. A particularly preferred flux includes about 60 wt. % CsF, about 30 wt. % AlF3, and about 10 wt. % Al2O3. The flux is preferably coated on the surface to be joined in a thickness of about 5 to 20 g/m2. Prior to placing in a mold, the surface of the component to be coated with flux is roughened to enhance adhesion of flux and metal thereto. Components suitable for use with the present invention include castings, extrusions or sheets of AA 6000 series wrought aluminum alloys. The molten aluminum alloy may be an Al-Mg-Si casting aluminum alloy.
The present invention includes a process for joining aluminum components. This process provides for joining of a component, such as a cast component (casting), an extruded member (extrusion) and sheet product, by directly casting a cast member in place onto the component. A cast joint reduces the cost associated with producing large aluminum structural assemblies. The components joined by cast joining may be made to less stringent tolerances, thereby eliminating the machining operations used to guarantee consistent fit and welds gaps. Costly assembly fixtures and other equipment such as welding power sources are not necessary. The labor of conventional welding processes is greatly reduced. In addition, the cast joining process of the present invention enables joints to be formed at locations where welding and other prior techniques are difficult to achieve.
The present invention includes the steps of 1) coating at least one surface of an aluminum alloy component with flux, 2) placing the flux coated component in a mold, 3) filling the mold with molten aluminum alloy and 4) allowing the molten metal to solidify whereby the molten metal solidifies as a casting on the component. The flux distributes itself closely between the surface of the component and metal to be joined, typically via capillary action. The liquidus of the flux is preferably less than the solidus of the metal of the component being joined. The flux removes oxides on the surface of the component and oxygen in the atmosphere adjacent the surfaces being joined. Hence, the flux must begin to melt at a temperature low enough to minimize oxidation of the parts be essentially molten at the time that the molten metal contacts the component to be joined, flow over both the surface to be joined and the molten metal to shield the component and the molten metal from oxidation, penetrate oxide films present on the component to be joined, and lower the surface tension between the solid metal of the component and the liquid (molten) metal to promote wetting.
The flux used in the present invention is preferably non-corrosive, non-hygroscopic, and generates minimal fumes during cast joining. A preferred flux for practicing the method of the present invention is a cesium fluoride composition. The flux preferably includes CsF, AlF3, and Al2O3, more preferably, about 60 wt. % CsF, about 30 wt. % AlF3 and about 10 wt. % Al2O3.
The flux of the present invention may be provided in a carrier such as water or alcohol and may be applied by dipping, brushing, spraying, or the like. The flux is preferably coated on the surface to be joined in a thickness of about 5 to 20 g/m2. Preferably, the surface of the component to be joined is roughened, such as by shot blasting, glass bead blasting, and cleaning with a wire brush. The surface may also be cleaned with a mild caustic etch solution and washed with acetone.
Components which may be joined via the method of the present invention may be formed from a metal which has a solidus above the liquidus of the molten (casting) metal. Suitable metals for the components to be joined include aluminum alloys such as aluminum Association (AA) alloys of the 6000 series, preferably AA 6061. The solidus of AA 6061 is 1140xc2x0 F., and the liquidus of AA 6061 is 1205xc2x0 F. The molten metal may be a casting alloy containing Al, Mg and Si, preferably AA A356. The solidus of AA A356 is 1007xc2x0 F., and the liquidus of A356 is 1135xc2x0 F.
Although the invention has been described generally above, the particular examples give additional illustration of the product and process steps typical of the present invention.